I'm not sure how they're handled internally, but async functions are no longer generators. For a while after generators and before async/await, generators were used as a polyfill.

Because of how similar they are, it would make sense to implement async/await using existing generator backends in the VM, right?

Not unless promises are implemented with generators. I dunno though, I feel like there's some overlap between the two concepts, so maybe they are?

Promises are just objects which represent an asynchronous value.

Async/await are coroutines: they have a shared context, stack, etc. They can be paused and resumed.

The implementation of async/await depends on the engine, but I'm pretty sure they use the same generator backend.

Babel and other transpilers convert async functions actual generator functions. By contrast, promises are a fairly simple runtime library.